An example of a device that is capable of transmitting a signal, commonly referred to as a transmitter, is shown schematically in FIG. 1. A signal, typically a radio-frequency signal, is generated by signal electronics 110, which may be arranged to modulate a carrier signal with a baseband signal, the carrier signal typically being a periodic waveform at a given frequency that may be transmitted and the baseband signal representing information to be transmitted. In FIG. 1 the signal electronics 110 pass a modulated signal to an amplifier 120, which may be a power amplifier. In certain arrangements a front-end module 130 may also be provided to provide processing prior to transmission from at least one antenna 140. These arrangements are often found in user equipment such as mobile communications devices that communicate via a network interface comprising a radio transceiver to a network of base stations connected to a telecommunications network.
Most amplifiers, for example power amplifier 120, have a linear range of operation and a non-linear range of operation. This is shown by example operating curve 205 in FIG. 2A. As input power is increased the amplifier moves from the linear range of operation to the non-linear range of operation. In the linear range of operation an output power is proportional to an input power as shown by line 210. In the non-linear range output power is a non-linear function of input power, wherein eventually output power reaches a saturation point 220. In the non-linear range spectral broadening of a radio-frequency signal may occur. An example of this is shown in FIG. 2B. In this case a radio-frequency signal has a specified range that is centered around a local oscillator (LO) frequency 250. This radio-frequency signal is amplified by amplifier 120 for transmission by antenna 140. In the example, non-linear characteristics of the amplifier 120 generate spectral components with frequencies in a range below 260L and above 260U the specified radio-frequency range 255 of the amplified signal. These spectral components are referred to as intermodulation products or distortions as they occur outside of specified modulation bands (such as range 255). If left unmodified, these distortions can lead to an increased bit error rate at a receiver of the transmitted signal. Transmission formats, such as wideband code division multiple access (WCDMA) used in 3rd generation (3G) systems or orthogonal frequency division multiplexing (OFDM) used in 4th generation (4G) systems such those based on Long Term Evolution (LTE) standards are vulnerable to nonlinear distortions as their signals have relatively high peak-to-average power ratios.
One method of reducing and/or avoiding non-linearities and their effects is to restrict operation of an amplifier to the linear range. This is sometimes referred to as “back-off” operation as it involves “backing off”, i.e. reducing, input power from a 3 dB operating point to an operating point marking the end of the linear range. For example, in FIG. 2A this is shown by arrow 225. However, this may decrease power efficiency for signals with relatively high peak-to-average power ratios and/or lead to degraded noise performance for current-mode modulators.
Another method of reducing and/or avoiding non-linearities and their effects is to “predistort” a signal before amplification to compensate for the non-linear characteristics of an amplifier. This is illustrated in FIG. 2C and is often also referred to as “linearizing” a signal. Chart 275 shows the operating characteristics of an amplifier, similar to chart 200, including linear and non-linear regions of operation. Based on these operating characteristics a signal is modified, typically to have non-linear characteristics as shown in chart 270, such that when the signal is amplified the result is a linear region of operation as shown in chart 280. Many different linearization techniques are available to predistort the signal. FIG. 2A shows an area of operation 230 that may be achieved, for example in addition to area 215, when using certain linearization techniques.
Typically, such existing linearization techniques require a trade-off between different operating requirements. It would be useful to provide an improved apparatus and method for predistorting or linearizing a signal.